Our research team will generate, validate, and deliver novel carrier-payload conjugates that are designed to selectively and potently eradicate chronic lymphocytic leukemia (CLL), the most common leukemia in the Western world, without affecting normal cells and tissues. CLL is an indolent yet incurable B-cell malignancy that afflicts approximately 150,000 patients and causes 4,500 deaths per year in the USA alone. There are currently no treatment options for CLL that allow for selective targeting of malignant B cells and that spare normal B cells and other normal cells and tissues. We hypothesize that the recently discovered Fc? receptor TOSO, on its own or in combination with other cell surface receptors that are selectively expressed in CLL, can mediate rapid and effective cellular entry of cytotoxic drugs for potent and specific therapeutic intervention in CLL. We propose two Specific Aims to rigorously test this hypothesis. In Aim 1 we will generate a series of molecularly defined carrier-payload conjugates that deliver highly cytotoxic drugs via the TOSO internalization pathway. These carrier-payload conjugates will be rigorously validated in vitro, ex vivo and in vivo and wil be assessed for their stability, specificity, anti-CLL activity, and toxicity. Throughout this campaign we will test conceptually novel biological and chemical components of carrier- payload conjugates. Whereas the studies of Aim 1 will optimize the potency of carrier-payload conjugates targeting TOSO, in Aim 2 we will improve the specificity of this drug delivery strategy. In particular, we will introduce a second specificity that is directed by an antibody fragment that binds with high specificity and avidity to the receptor tyrosine kinase ROR1, which we have shown is selectively expressed in CLL cells but not in normal B and T cells nor in other normal cells or tissues. Furthermore, we will test the utility of other CLL cell surface receptors for dual targeting in combination with TOSO. The stability, specificity, avidity, anti-CLL activity and toxicity of these bispecific carrier-payload conjugates will be tested using validated in vitro ex vivo, and in vivo models of CLL. Collectively, our research team will deliver a series of highly specific, safe, and potent reagents that will lay the foundation for further preclinical and clinicl investigations in the treatment of CLL. Notably, the conceptually novel biological and chemical components coming from these campaigns can be easily and rapidly exploited to generate other carrier-payload conjugates well beyond the realm of CLL.